1. Field of the Invention
The present invention relates generally to a holographic memory and to an optical information recording/reproducing apparatus utilizing the holographic memory, and more particularly to a hologram recording apparatus and a method therefor for recording a signal which are free from deterioration when it is reproduced later.
2. Description of the Related Art
Conventionally, a holographic memory system is known as a digital recording system which applies the principle of holography. In the following, a holographic memory system will be generally described with reference to FIG. 1.
In FIG. 1, an encoder 25 converts digital data to be recorded in a holographic memory 1 to a light/dark dot pattern image on a plane, and rearranges the dot pattern image into a data array of, for example, 480 bits in the vertical direction and 640 bits in the horizontal direction (480xc3x97640) to generate sequence data in unit pages. This data is sent to a spatial light modulator (SLM) 15, such as a transmission-type TFT liquid crystal display (LCD) panel, by way of example.
The spatial light modulator 15, which has a modulation processing unit corresponding to the unit page composed of 480 bits in the vertical direction and 640 bits in the horizontal direction (480xc3x97640), optically modulates a light beam irradiated thereto to spatial light on/off signal in accordance with the unit page sequence data from the encoder 25, and sends the modulated signal beam or signal light to a lens 16. More specifically, the spatial light modulator 15 passes therethrough a signal beam corresponding to a logical value xe2x80x9c1xe2x80x9d in the unit page sequence data which is an electrical signal, and blocks the signal beam corresponding to a logical value xe2x80x9c0xe2x80x9d in the unit page sequence data to achieve photoelectric conversion in accordance with respective bit contents of the unit page data, thereby generating a signal beam which is modulated as signal light of the unit page sequence.
The signal light is incident on the holographic memory 1 through a lens 16. In addition to the signal light, the holographic memory 1 is also irradiated with reference light at an incident angle xcex2 from a predetermined base line orthogonal to the optical axis of the beam of the signal light.
The signal light and the reference light interfere with each other in the holographic memory 1 to produce interference fringes which are stored in the holographic memory 1 as a refractive index grating or hologram to record the data. Also, the holographic memory 1 provides for three-dimensional data recording by entering the reference light thereto with a different incident angle xcex2 to record a plurality of two-dimensional planar data in an angle multiplex scheme.
For reproducing recorded data from the holographic memory 1, the reference light only is directed into the holographic memory 1 at the same incident angle xcex2 as recording, toward the center of a region in which the signal light beam and the reference light beam intersect. In other words, unlike recording, the signal light is not directed. In this way, diffraction light from the interference fringes recorded in the holographic memory 1 is transmitted to a CCD (Charge Coupled Device) 20, which functions as a photodetector, through a lens 19. The CCD 20 converts light and dark of the incident light to the intensity of an electrical signal to produce an analog electrical signal having a level in accordance with the luminance of the incident light, which is output to a decoder 26. The decoder 26 compares this analog signal with a predetermined amplitude value (slice level) to reproduce corresponding data xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d.
Since the holographic memory records data in two-dimensional planar data sequences as described above, the angle multiplex recording can be accomplished by changing the incident angle xcex2 of the reference light. Stated another way, a plurality of two-dimensional planes as recording units can be defined in the holographic memory by changing the incident angle xcex2 of the reference light, with the result that three-dimensional recording is enabled.
Conventionally, for a rewritable holographic memory 1 utilizing the photo-refractive effect, Fe-added lithium niobate (LiNbO3, or abbreviated as xe2x80x9cLNxe2x80x9d) single crystals are used as recording materials, while a wavelength of 532 nm, which is a second harmonic of an Nd:YAG laser, is used as recording light. In this conventional recording scheme (called the xe2x80x9cconventional single-color recording schemexe2x80x9d), corresponding to interference fringes formed from signal light and reference light, which are recording light, electrons are excited from an Fe2+ state to a conduction band in light regions of the interference fringes, undergo a photo-refractive process, and are finally trapped to an Fe3+ state to complete the storage.
However, the conventional single-color recording scheme implies a problem that reproduction light gradually erases the recorded hologram when a signal is read from the hologram (which is so called reproduction deterioration). The medium has a sensitivity to light of one wavelength that is used at the time of recording and reproduction. In the single color hologram, recorded information is electrons trapped at the trap level (storage center) which is produced by Fe. That is, every time reproduction is performed, electrons are gradually excited to the conduction band from the trap level, thereby erasing the stored information. According to the conventional holographic memory, when signals are read from a hologram recorded there, reproduction light gradually erases the hologram, so that the reproduction deterioration occurs.
On the other hand, a two-color hologram scheme is known as a recording scheme which suffers from less reproduction deterioration.
The two-color hologram recording is characterized in that a hologram is recorded by simultaneously irradiating other light called xe2x80x9cgate lightxe2x80x9d (at wavelength xcex2), in addition to recording light (reference light and signal light at wavelength xcex1) for forming the hologram. The gate light acts to develop a recording sensitivity at the wavelength (xcex1) of the recording light only during the irradiation of the gate light. Such a characteristic is based on carriers temporarily formed by the irradiated gate light at a relatively shallow energy state called an xe2x80x9cintermediate excitation statexe2x80x9d within a portion of the crystal irradiated with the gate light. The carriers at the intermediate excitation state are excited by the recording light (a spatial light/dark pattern corresponding to interference fringes formed by the reference light and the signal light), and finally accumulated in the form of a variable density distribution of the carriers corresponding to the interference fringes at a deep trap state. The latter process of the two-color hologram scheme, which is called the xe2x80x9cphoto-refractive effect,xe2x80x9d is in principle the same process as the single-color hologram. For example, with the two-color hologram recording scheme using crystals which are processed to be reduced to LiNbO3 with no additive component or with Fe added thereto, and have a composition close to the stoichiometry (abbreviated as xe2x80x9cSLNxe2x80x9d) (H. Guenther, R. M. Macfarlane, Y. Furukawa, K. Kitamura, R. Neurgaonkar; xe2x80x9cTwo-color holography in reduced near-stoichiometric lithium niobatexe2x80x9d, Appl. Opt. Vol. 37, pp. 7611-7623 (1998)), the lifetime of carriers at the intermediate excitation state (metastable state) can be extended from microseconds to seconds, thereby making it possible to use a continuous oscillating laser having relatively small power for recording.
While the two-color hologram recording scheme requires a reduction of a recording material to increase the PR center density (bipolaron-polaron mechanism), this results in a lower density of Fe3+ (trivalent) to degrade the transparency of the material itself. Also, since the light sensitivity is still insufficient for a practical level, there has been a need for development of a hologram recording scheme which provides a higher sensitivity.
Further, in the two-color holographic recording scheme, if the lifetime of the intermediate excitation state is so long that carriers exist at that state even after writing, carriers excited upon reading reflect the electric field to recombine. As a result, since such carriers cancel the previously formed spatial electric field, the diffraction efficiency is significantly reduced.
It is therefore an object of the present invention to provide a hologram recording apparatus which is capable of increasing the light sensitivity, and for reducing the signal deterioration during reproduction to exhibit good data indestructibility.
The present invention provides a hologram recording apparatus for directing interferable signal light and reference light into a hologram recording medium to record an information signal carried by the signal light, wherein hologram recording medium is sensitive to a first light at a first wavelength in an ultraviolet or short-wavelength visible light band to develop light induced absorption. The apparatus includes means for irradiating the hologram recording medium with the first light; and means for irradiating the hologram recording medium with signal light and reference light at a second wavelength longer than the first wavelength after the first light is irradiated.
In an aspect of the present invention, said hologram recording medium comprises a photo-refractive material selected from a group consisting of a lithium niobate (LiNbO3) single crystal which includes a rare earth element, and has a molar fraction of [Li2O]/([Li2O]+[Nb2O5]) in a range of 0.482 to 0.505, or a lithium tantalate (LiTaO3) single crystal which includes a rare earth element, and has a molar fraction of [Li2O]/([Li2O]+[Ta2O5]) in a range of 0.482 to 0.505.
In another aspect of the present invention, the molar fraction of [Li2O]/([Li2O]+[Nb2O5]) of lithium niobate lies in a range of 0.490 to 0.505 or the molar fraction of [Li2O]/([Li2O]+[Ta2O5]) of lithium tantalate lies in a range of 0.490 to 0.505.
In another aspect of the present invention, said rare earth element is Tb, and is doped by an amount ranging from 10 weight ppm to 1000 weight ppm.
In a further aspect of the present invention, said photo-refractive material simultaneously includes Fe or Mn in addition to Tb.
In a still further aspect of the present invention, said photo-refractive material includes Fe or Mn by an amount ranging from 1 weight ppm to 500 weight ppm.
Also, the present invention provides a hologram recording method for directing interferable signal light and reference light into a hologram recording medium to record an information signal carried by the signal light, said hologram recording medium being sensitive to a first light at a first wavelength in an ultraviolet or short-wavelength visible light band to develop light induced absorption, said method comprising the steps of:
irradiating said hologram recording medium with said first light; and
irradiating said hologram recording medium with signal light and reference light at a second wavelength longer than said first wavelength after said first light is irradiated.